This invention relates generally to protective coatings for metal substrates. More particularly, it is directed to improved thermal barrier coatings applied to metals designed for high temperature applications.
Superalloys are often the materials of choice for components intended for high-temperature environments. As an example, turbine blades and other parts of turbine engines are often formed of nickel-based superalloys because they need to maintain their integrity at temperatures of at least about 1000xc2x0 C.-1150xc2x0 C. Protective coatings, often referred to as thermal barrier coatings or xe2x80x9cTBCxe2x80x9ds, effectively increase the operating temperature of turbine components by maintaining or reducing the surface temperature of the alloys used to form the various engine components.
Most TBC""s are ceramic-based, such as a material like yttria-stabilized zirconia. For a jet engine, the coatings are applied to various surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. Usually, the TBC ceramics are applied to an intervening bond layer which has been applied directly to the surface of the metal part. The bond layer is often critical for improving the adhesion between the metal substrate and the TBC. Bond layers are usually formed from a material like xe2x80x9cMCrAlYxe2x80x9d, where xe2x80x9cMxe2x80x9d represents a metal like iron, nickel, or cobalt.
The term xe2x80x9csuperalloyxe2x80x9d is usually intended to embrace complex cobalt- or nickel-based alloys which include one or more other elements such as aluminum, chromium, tungsten, molybdenum, titanium, and iron. The quantity of each element in the alloy is carefully controlled to impart specific characteristics, e.g., mechanical properties such as high-temperature strength. Aluminum is a particularly important component for many superalloys, because of its function in the precipitation-strengthening of the alloy.
If the superalloy is exposed to an oxidizing atmosphere for an extended period of time, it can become depleted in aluminum. This is especially true when the particular superalloy component is used at the elevated temperatures described above. The aluminum loss can occur by way of various mechanisms. For example, aluminum can diffuse into the bond coat, be consumed during oxidation of the bond coat, or be consumed during oxidation at the bond coat/substrate interface. The last-mentioned mechanism is particularly severe in porous bond coats, such as air plasma-sprayed (APS) bond coats. Aluminum-loss from the substrate is accelerated if the TBC or bond coat is removed during the service life of the component.
Since loss of aluminum can be detrimental to the integrity of the superalloy, techniques for countering such a loss have been investigated. At elevated temperatures, the substrate can be partially xe2x80x9creplenishedxe2x80x9d with aluminum which diffuses from an adjacent MCrAlY-type bond coat. However, the amount of aluminum diffusion into the substrate from the bond coat is usually insufficient.
One method for increasing the aluminum content of the superalloy in its surface region is sometimes referred to in the art as xe2x80x9caluminidingxe2x80x9d. In such a process, aluminum is introduced into the substrate by a variety of techniques. In the xe2x80x9cpack aluminidingxe2x80x9d process, the substrate is immersed within a mixture or pack containing the coating element source, filler material, and halide energizer. At temperatures about 850-1100xc2x0 C., chemical reactions within the mixture yield an aluminum-rich vapor which condenses onto the substrate surface, and subsequently diffuses into the substrate.
While aluminiding successfully provides aluminum to the substrate and substrate-bond coat interface, there are some disadvantages associated with such a technique. For example, the resulting high-aluminum surface layer can be brittle. Deposition of an overlay bond coat on a brittle surface can sometimes be difficult.
It should thus be apparent that new methods for increasing the aluminum content of the superalloy surface and thereby increasing its oxidation life would be welcome in the art. These methods should prevent the formation of a brittle layer between the substrate and any subsequently-applied layer. Moreover, the new methods should result in a surface which is very amenable to deposition of subsequently-applied layers. It would also be very advantageous for the new methods to be capable of providing aluminum to a bond coat layer, to compensate for aluminum consumed in the bond coat by way of oxidation.
In one embodiment, the invention is directed to a method for providing a protective coating on a metal-based substrate, comprising the following step:
(a) applying an aluminum-rich mixture to the substrate to form a discontinuous layer of aluminum-rich particles in a matrix of metallic bond coat alloy, wherein the amount of aluminum in the particles exceeds the amount of aluminum in the metallic bond coat alloy by about 0.1 atomic % to about 40 atomic %, and wherein the total amount of aluminum in the mixture is in the range of about 10 atomic % to about 50 atomic per cent.
In a second embodiment, the invention is directed to a method for providing a protective coating on a metal-based substrate, comprising the following steps:
(a) applying an aluminum-rich mixture to the substrate to form a discontinuous layer of aluminum-rich particles in a matrix of metallic bond coat alloy, wherein the amount of aluminum in the particles exceeds the amount of aluminum in the metallic bond coat alloy by about 0.1 atomic % to about 40 atomic %, and wherein the total amount of aluminum in the mixture is in the range of about 10 atomic % to about 50 atomic %; and then
(b) applying at least one coating layer over the discontinuous layer of aluminum-rich particles.
Aluminum diffuses from the aluminum-rich layer into the superalloy substrate, as discussed below. The discontinuous nature of the aluminum-rich layer prevents embrittlement.
In preferred embodiments, substantially all of the aluminum-rich material comprises non-oxide particles. Moreover, in many preferred embodiments, the aluminum rich layer is formed of two components. Component (I) usually comprises particles of aluminum and a second metal, such as nickel, while component (II) usually comprises particles of an alloy of the formula MCrAlY, where M is a metal like Fe, Ni, Co, or mixtures which comprise any of the foregoing. The aluminum-rich layer can be applied by plasma spray techniques, such as air plasma spray or vacuum plasma spray, or by high velocity oxygen fuel (HVOF).
In some embodiments, the layer formed with the aluminum-rich mixture is heat-treated after being applied, to allow diffusion of aluminum into the superalloy. Moreover, in certain embodiments, a conventional metallic bond layer is applied over the aluminum-rich layer, prior to deposition of a thermal barrier coating. The heat treatment mentioned above can alternatively be carried out after deposition of the thermal barrier coating.
Another aspect of this invention is directed to an article, comprising:
(i) a metal-based substrate; and
(ii) an aluminum-containing layer over the substrate, comprising a discontinuous layer of aluminum-rich particles. In many preferred embodiments, the article may also include a thermal barrier coating disposed over the aluminum-containing layer.
As mentioned previously, the aluminum-containing layer can be formed from a mixture of a component based on particles of aluminum and nickel, along with a component based on a conventional MCrAlY alloy. Moreover, a metallic bond layer can be disposed between the aluminum-containing layer and the thermal barrier coating.
Further details regarding the various aspects of this invention are provided in the remainder of the disclosure.
A variety of metals or metal alloys can be used as the substrate for the present invention. The term xe2x80x9cmetal-basedxe2x80x9d in reference to substrates disclosed herein refers to those which are primarily formed of metal or metal alloys, but which may also include some non-metallic components, such as ceramics, intermetallic phases, or intermediate phases. Usually, the substrate is a heat-resistant alloy, including superalloys which typically have an operating temperature of up to about 1100-5000xc2x0 C. They are described in various references, such as U.S. Pat. Nos. 5,399,313 and 4,116,723, both incorporated herein by reference. Illustrative nickel-base superalloys are designated by the trade names Inconel(copyright), Nimonic(copyright), Rene(copyright) and Udimet(copyright). The type of substrate can vary widely, but it is often in the form of an engine part, such as a turbine blade (bucket), a turbine nozzle guide vane, or a combustor liner. As another example, the substrate may be the piston head of a diesel engine, or any other surface requiring a heat-resistant barrier coating. In some instances, the substrate thickness can be quite small, for example, less than about 0.25 cm. Thermal protection of thin-walled superalloy components is often a critical task.
As mentioned above, an aluminum-rich mixture is applied to the substrate. Conventional pretreatment steps may be taken prior to deposition of the aluminum-rich mixture, e.g., cleaning of the substrate surface; grit blasting to remove debris and to roughen the surface; and the like. The amount of aluminum in the aluminum-rich mixture will depend in part on the amount of aluminum intended for diffusion from the layer into the superalloy substrate and into any subsequently-applied bond coat layer. Those amounts will in turn depend on the projected loss of aluminum from the substrate and bond coat layers during exposure to oxidizing atmospheres and high temperature. The approximate, projected loss of aluminum can be determined by, first, exposing the substrate and bond coat materials in the absence of the aluminum-rich layer of this invention to an oxidizing environment under selected time and temperature schedules. The microstructures of the materials can then be examined, using various devices, such as a scanning electron microscope (SEM), equipped with an energy-dispersive X-ray detector. Such devices are capable of quantifying the aluminum loss from the bond coat and from the surface region of the substrate. The xe2x80x9csurface regionxe2x80x9d is defined herein as the region extending from the bond coat-substrate interface to about 600 microns into the substrate.
In general, the amount of aluminum in the aluminum-rich layer will be large enough to compensate for any projected loss of aluminum from the substrate or an adjacent bond coat layer, but small enough to prevent the formation of a continuous, brittle, aluminum-containing layer, as described previously. Parameters for aluminum content, based on atomic percentages, were provided above. In terms of weight percentages, the amount of aluminum in the mixture is often in the range of about 4% by weight to about 32% by weight. In preferred embodiments, the amount of aluminum is in the range of about 10% by weight to about 20% by weight. In some especially preferred embodiments, the amount of aluminum is in the range of about 12.5% by weight to about 19% by weight.
The aluminum-rich mixture can be obtained from a variety of sources. In general, any aluminum-containing material which can release aluminum into adjacent surfaces or layers at elevated temperatures, such as above about 600xc2x0 C., would be suitable, as long as the material does not react adversely with the substrate material or the material which forms any subsequently-applied bond coat. Non-limiting examples of aluminum-releasing compounds which are suitable for most embodiments include those based on aluminum and nickel, such as NiAl and Ni3Al; those based on aluminum and titanium, such as TiAl and Ti3Al; those based on aluminum and iron, such as FeAl and Fe3Al; those based on aluminum and cobalt, such as CoAl; and those based on aluminum and zirconium, such as Zr3Al. Such materials are usually commercially available, or can be prepared without undue difficulty.
In some preferred embodiments, the aluminum-rich mixture is based on at least two components. Component (I) can be a compound based on aluminum and a second metal, as mentioned above. In general, the level of aluminum for this component is in the range of about 20 atomic % to about 55 atomic %, based on the total atoms of aluminum and the second metal. In regard to the exemplary aluminum-releasing compounds listed above, this range generally corresponds to a weight-based range of about 8% by weight to about 37% by weight aluminum.
Moreover, in preferred embodiments, the second metal is nickel to constitute the compounds NiAl or Ni3Al. In the case of NiAl or Ni3Al, conversion of a preferred atomic percentage-based range to a weight percentage-based range results in a range of about 13% by weight aluminum to about 31.5% by weight aluminum. Those skilled in the art understand that the proportion of atoms in such compounds may vary somewhat from the stoichiometric proportions, but are still within the scope of the formula-designation, NiAl or Ni3Al.
In the embodiments based on at least two components, component (II) is usually a conventional material used for bond coats. Examples include alloys of the formula MCrAlY, where M is selected from the group consisting of Fe, Ni, Co, and mixtures thereof. Many of these types of alloys have a general composition, by weight, of about 17.0-23.0% chromium; about 4.5-12.5% aluminum; and about 0.1-1.2% yttrium; with M constituting the balance.
For the two-component embodiment, the relative proportions of components (I) and (II) will depend in part on some of the factors described above, regarding aluminum depletion and replenishment. In general, the level of component (I) is at least about 1% by volume, based on the total volume of components (I) and (II). In preferred embodiments, the level of component (I) is in the range of about 5% by volume to about 50% by volume, while in especially preferred embodiments, the level of component (I) is in the range of about 15% to about 50% by volume.
It should be understood that components (I) and (II) could alternatively be expressed in terms of weight percentages, as long as the densities of particles for each component are taken into account. As an example, if component (I) is NiAl, the particle density is approximately 5.8 g/cm3. If component (II) is NiCrAlY (having a nominal composition as follows: 68 wt. % Ni, 22 wt. % Cr, 9 wt. % Al, and 1 wt. % Y), its particle density is approximately 8.0 g/cm3. In such a case, 1% by volume converts to about 1% by weight, based on total weight; 5%-50% by volume converts to about 4%-42% by weight; and 15%-50% by volume converts to about 11%-42% by weight. In similar fashion, the weight levels of other materials for components (I) and (II) could be calculated, based on their volume levels and densities.
The thickness of the aluminum-rich layer will also depend on a variety of factors, such as the type of substrate being protected; the presence or absence of a subsequently-applied bond coat; and the desired amount of aluminum content. In those instances in which no bond coat is to be subsequently applied, the aluminum-rich layer will usually have a thickness in the range of about 25 microns to about 400 microns, and preferably, in the range of about 100 microns to about 300 microns. If a bond coat is to be subsequently applied, the aluminum-rich layer will often have a thickness in the range of about 25 microns to about 300 microns, and preferably, in the range of about 50 microns to about 200 microns.
The aluminum-rich mixture can be applied by a variety of techniques. Usually, a plasma spray technique or HVOF is preferred. For the plasma spray technique, an electric arc is typically used to ionize various gases, such as nitrogen, argon, helium, or hydrogen, to temperatures of about 8000xc2x0 C. or greater. When the process is carried out in an air environment, it is often referred to as air plasma spray or xe2x80x9cAPSxe2x80x9d. The gases are expelled from an annulus at high velocity, creating a characteristic thermal plume. Powder material is fed into the plume, and the melted particles are accelerated toward the substrate being coated. Plasma-formed layers usually have a very rough surface, which enhances their adhesion to a subsequently-applied thermal barrier coating.
As mentioned earlier, the layer of aluminum-rich particles is discontinuous. In other words, the particles of the aluminum-rich material are not in continuous contact with each other. Instead, these discrete particles are usually separated from each other by particles of another metal-based materialxe2x80x94usually the bond coat-type material MCrAlY. A minor portion of the aluminum-rich layer might be considered to be contiguous. xe2x80x9cContiguityxe2x80x9d is a term which relates to the continuous nature of a layer of particles. Contiguity is defined herein as the fraction of total interfacial area of one phase that is shared by particles of the same phase.
In general, contiguity is a measure of the average degree of contact between aluminum-rich particles in the aluminum-rich mixture. The fraction of total interfacial area of one phase shared by particles of the same phase ranges from 0% to 100% as the distribution of aluminum-rich phase changes from a completely dispersed structure to a fully continuous structure. In some embodiments, the contiguity fraction for the aluminum-rich particles is less than about 65%. In other words, less than about 65% of the interfacial area of each aluminum-rich particle in the layer is in contact with an adjacent aluminum-rich particle. Such a layer is said to be xe2x80x9cdiscontinuousxe2x80x9d according to the present description of this invention. In preferred embodiments, the contiguity fraction for the aluminum-rich particles is less than about 40%. In especially preferred embodiments, the contiguity fraction for the aluminum-rich particles is less than about 25%.
The discontinuous nature of the aluminum-rich layer inhibits the embrittlement which characteristically occurs with continuous layers of aluminum-rich materials. In the case of the two-component aluminum-rich layer, the aluminum-rich particles of component (I) would be separated from each other by the particles of component (II), e.g., the MCrAlY matrix.
In some embodiments and especially in the case of a superalloy substrate, the aluminum-rich layer is heat-treated after being deposited. The heat treatment promotes the diffusion of aluminum into the substrate. It is usually carried out in an oxygen-free environment, to avoid oxidation of the layer. For example, the heat treatment could be carried out under vacuum, in a hydrogen atmosphere, or in an inert gas atmosphere.
The particular conditions for the heat treatment will depend on a variety of factors, such as the thickness and particular composition of the aluminum-rich layer; the type of substrate; the projected loss of aluminum from the substrate and/or from any subsequently-applied bond coat layer; the projected thermal exposure for the substrate in service; the required strength for the substrate alloy; and economic considerations. In general, the aluminum-rich layer is usually heat-treated at a temperature in the range of about 925xc2x0 C. to about 1260xc2x0 C., for a time period of about 15 minutes to about 16 hours. In some preferred embodiments, the heat treatment is carried out at a temperature in the range of about 980xc2x0 C. to about 1150xc2x0 C., for a time period of about 1 hour to about 8 hours. As described below, the heat treatment can alternatively be carried out at a later stage, such as after the deposition of additional layers of various materials.
As mentioned above, a metallic bond layer or xe2x80x9cbond coatxe2x80x9d may be applied over the aluminum-rich layer. The bond layer promotes adhesion between the coated substrate and a thermal barrier coating which is applied at a later stage. The bond layer also provides further protection to the substrate, especially in those instances in which the part may be exposed to damaging environments. Exemplary bond coat layers are known in the art and described, for example, in U.S. Pat. Nos. 5,419,971 and 5,043,138, both incorporated herein by reference. The bond coat usually has a thickness in the range of about 50 microns to about 500 microns, and preferably, in the range of about 50 microns to about 375 microns. Preferred bond coatings for this invention have the formula MCrAlY, where xe2x80x9cMxe2x80x9d can be various metals or combinations of metals, such as Fe, Ni, or Co. In many instances, xe2x80x9cMxe2x80x9d is preferably nickel.
The bond coat may be applied by a variety of conventional techniques, such as plasma spray or other thermal spray deposition methods, such as HVOF, detonation, or wire spray; CVD (chemical vapor deposition); or combinations of plasma spray and CVD techniques. Sometimes, it may be convenient to employ the same deposition technique used to apply the aluminum-rich layer.
In many instances, a plasma spray technique is the preferred method for depositing the bond layer. Specific, non-limiting examples are low pressure plasma spraying and air plasma spraying. Details regarding plasma spraying can also be found, for example, in Kirk-Othmer""s Encyclopedia of Chemical Technology, 3rd Edition, Vol. 15, (1981) and Vol. 20 (1982); in Ullmann""s Encyclopedia of Industrial Chemistry, Fifth Edition; Volume A6, VCH Publisher (1986); in Scientific American, H. Herman, September, 1988; and in U.S. Pat. 5,384,200, incorporated herein by reference. One of ordinary skill in the art can easily become familiar with various process details which may be relevant: spray distances; selection of the number of spray-passes, gas temperature; powder feed rate; powder particle size and size distribution; heat treatment after deposition; or any other type of treatment after deposition.
A heat treatment as described earlier may be carried out after deposition of the bond coat layer. This heat treatment could replace the earlier heat treatment, or it could be an additional treatment. The conditions for this heat treatment would be similar to those described earlier.
The thermal barrier coating is then applied over the bond coat, or over the aluminum-rich layer in those instances in which a bond coat is not employed. Usually, the TBC is zirconia-based, as mentioned previously. As used herein, xe2x80x9czirconia-basedxe2x80x9d embraces ceramic materials which contain at least about 50% zirconia. In preferred embodiments, the zirconia is chemically stabilized by being blended with a material such as yttrium oxide, calcium oxide, magnesium oxide, cerium oxide, scandium oxide, or mixtures of any of those materials. In one specific example, zirconia can be blended with about 1% by weight to about 20% by weight yttrium oxide (based on their combined weight), and preferably, from about 3%-10% yttrium oxide.
The thermal barrier coating can be applied by a variety of techniques, one being electron beam physical vapor deposition (EB-PVD). In some preferred embodiments, the thermal barrier coating is applied by plasma-spray techniques, which were described previously. Plasma spray systems are especially suited for coating large parts, with good control over the thickness and uniformity of the coatings. In general, the thickness of the thermal barrier coating is in the range of about 75 microns to about 2000 microns. The most appropriate thickness depends in large part on the end use of the component.
After the thermal barrier coating has been applied, a heat treatment may be carried out, in addition to or in lieu of either of the earlier heat treatments. The conditions for this heat treatment are usually similar to those described earlier, although additional factors are taken into consideration, such as the thickness and composition of the thermal barrier coating. In preferred embodiments, the heat treatment at this stage will be carried out at a temperature in the range of about 980xc2x0 C. to about 1210xc2x0 C., for a time period of about 15 minutes to about 16 hours.
As mentioned earlier, a specific heat treatment need not be carried out in some embodiments of this invention. For example, a component such as a turbine engine would eventually be exposed to elevated temperatures, such as about 750xc2x0 C. to about 1150xc2x0 C., during its service life. Such thermal exposure would sometimes be sufficient to promote the diffusion of aluminum from the aluminum-rich layer into the substrate and any adjacent bond coat. The in-service heat treatment can occur as a supplement to one or more heat treatments carried out earlier, as discussed above.
It should be apparent from the discussion set forth above that another aspect of this invention is directed to a metal article provided with a protective coating, comprising:
(i) a metal-based substrate; and
(ii) an aluminum-containing layer over the substrate, comprising a discontinuous layer of aluminum-rich particles.
In many situations, a coating layer such as a bond coat or thermal barrier coating or both (component (iii)) is applied over the aluminum-containing layer, as described previously.
The amount of aluminum in layer (ii), as applied, is usually in the range of about 4% by weight to about 32% by weight. In preferred embodiments, the amount of aluminum is in the range of about 10% by weight to about 20% by weight. In some especially preferred embodiments, the amount of aluminum is in the range of about 12.5% by weight to about 19% by weight, as described previously. The thickness of layer (ii) is usually in the range of about 25 microns to about 400 microns.
In some embodiments, a metallic bond layer is disposed between layer (ii) and the thermal barrier coating layer (iii). The bond layer usually comprises an alloy of the formula MCrAlY, as described above.
Very often, the metal-based substrate is a superalloy, such as a nickel-based superalloy. In those instances, the thermal barrier coating is often zirconia-based. The superalloy may be a turbine engine component, for example. The presence of the aluminum-rich layer provides critical aluminum replenishment to both the substrate and any bond coat that has been applied before the deposition of the TBC. This replenishment in turn enhances the oxidation-resistance of the component.